The present invention relates to an electrolytic solution for lithium batteries. More specifically, the present invention relates to a method of removing water and free acids from an electrolytic solution for lithium batteries and to an electrolytic solution having a low water content and a low free acids content.
For lithium batteries, a non-aqueous electrolytic solution comprising a lithium electrolyte such as lithium hexafluorophosphate, LiPF6, in a non-aqueous organic solvent is used as an electrolytic solution. It is difficult to completely remove water contained in the solvent or in the electrolyte as an impurity, and water may further be introduced during storage of the electrolytic solution or a filling process of the electrolytic solution in batteries.
Further, a trace amount of free acids may be contained as impurities. Especially when an electrolyte which can be hydrolyzed or thermally decomposed with ease, such as LiPF6 or the like, is used, hydrofluoric acid is produced by hydrolysis with a trace amount of water or thermal decomposition due to dissolution heat. This hydrofluoric acid not only decreases a capacity of a battery or degrades charge-discharge cycle characteristics, but also corrodes the inside of the battery.
As methods of removing water in an electrolytic solution, the use of metal oxide such as molecular sieves, phosphorus pentaoxide, activated alumina, and calcium oxide is described in Japanese Patent Application Laid-Open S59-9874; the use of molecular sieves of a lithium ion type is described in Japanese Patent Application Laid-Open S59-81869; and the use of activated alumina is described in Japanese Patent Publication H3-49180.
Meanwhile, to remove free acids, there are known the following methods: a method where acids are removed by adsorption with an adsorbent, such as aluminum oxide, contained in a battery, as described in Japanese Patent Application Laid-Open H4-28437 and Japanese Patent Application Laid-Open H5-315006; a method where acids are removed by distillation; a method where acids are removed by ammonium salt dissolved in an electrolytic solution, as described in Japanese Patent Application Laid-Open H3-119667; a method where acids are removed by neutralization with an alkaline agent such as lithium hydroxide and lithium hydride, as described in Japanese Patent Application Laid-Open H4-282563; and use is made of a metal fluoride, as described in Japanese Patent Application Laid-Open H8-321326.
However, the methods where water and free acids are removed by a solid powdery adsorbent contained in a battery are less preferred, because a design of the battery should be modified. The adsorption method with molecular sieves or the like has only a little effect of removing water or the like, when conducted alone, and an additional process for separating and recovering the used adsorbent is required.
Japanese Patent Application Laid-Open H1-286262 discloses a method of removing free acids by adding an organic lithium compound such as pentafluorophenyllithium to an electrolytic solution. The present inventors have found that additional generation of free acids is suppressed only in a short period of time.
Thus, the purpose of the present invention is to provide a method of removing water and free acids simultaneously from an electrolytic solution without a need of modifying designs of batteries and without separating and recovering an adsorbent used. Another purpose of the present invention is to provide a method where the effect of suppressing additional generation of free acids is maintained for a prolonged period of time. Still another purpose of the present invention is to provide an electrolytic solution where a water content and a free acids content are both low and to provide a lithium battery comprising the electrolytic solution.
The present invention is a method of preparing an electrolytic solution for a lithium battery, comprising dissolving a lithium electrolyte in a solvent comprising at least one organic solvent, characterized in that the method comprises steps of
(a) leading an inert gas through the solvent having a water content of 100 ppm or lower under heating of the solvent to vaporize water together with the solvent to thereby reduce the water content of the solvent, and
(b) dissolving the lithium electrolyte in the solvent while maintaining a temperature of the solvent at 20xc2x0 C. or lower.
Preferably, the following step (c) follows after step (b),
(c) incorporating at least one lithium compound in the electrolytic solution, said lithium compound being selected from the group consisting of lithium amide compounds represented by the formula, LiNR1R2, lithium imide compounds represented by the formula, Li2NR3, lithium borohydride and derivatives thereof represented by the formula, LiBR4R5R6R7, organic lithium compounds represented by R8Li, lithium alkoxides represented by R9OLi, and lithium aluminum hydride and derivatives thereof represented by LiAlR10R11R12R13, wherein each of said R1-R3 independently represents hydrogen or a hydrocarbon residue.
More preferably, each of said R1-R13 is at least one independently selected from the group consisting of hydrogen, alkyl, aryl, and allyl.
The present invention also relates to an electrolytic solution for a lithium battery prepared by incorporating a lithium electrolyte in a solvent comprising at least one organic solvent, characterized in that the electrolytic solution has a water content of 3 ppm or lower and a free acids content, converted as hydrofluoric acid, of less than 1 ppm.
Further, the present invention relates to a lithium battery comprising the electrolytic solution obtainable by the method according to the present invention.
The present method comprises (a) a step of leading an inert gas through an organic solvent having a water content of at most 100 ppm at a room temperature under heating of the organic solvent. If the initial water content of the organic solvent is higher than 100 ppm, a larger amount of the inert gas is required to flow under heating, which is not preferred in terms of time and costs. To make the water content 100 ppm or less, any method may be used such as adsorption with an adsorbent such as molecular sieves, usual distillation under atmospheric pressure or distillation under a sub-atmospheric pressure, or purging with an inert gas. The water content can be determined by, for example, a Karl-Fisher method which will be described later in the specification.
Examples of the inert gas used in the present invention include nitrogen gas, helium gas, and argon gas, among which nitrogen gas is preferred in costs. Preferably, the inert gas contains substantially no water so as to show a dew point of xe2x88x9240xc2x0 C. or lower, preferably xe2x88x9260xc2x0 C. or lower.
The inert gas is led through a tube resistant to the organic solvent, such as a glass tube and a stainless tube. A flow rate may be set, depending on the amount of the solvent to be treated and a size of a container, but typically may be in the range of from 3 to 5 liters/minute for treating about 4 liters of the organic solvent.
By leading an inert gas through the organic solvent, the water content is made preferably 60 ppm or lower. Practically, the water content of from 40 to 60 ppm can be achieved. To make the water content less than 40 ppm, a larger volume of the inert gas and a longer treatment time are required, which may be disadvantageous in costs.
Examples of the organic solvent used in the present invention include dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, ethylene carbonate, methylethyl carbonate, propylene carbonate, xcex3-butyrolactone, sulfolane, tetrahydrofuran, 2-metylhydrofuran, dimethyl sulfoxide, dioxolan, dimethylformamide, acetonitrile or a mixture of two or more of them. Dimethyl carbonate and/or propylene carbonate are preferably used for their dielectric constants and viscosities.
When a mixture of two kinds of the organic solvents is used in the present invention, it is preferred to use a combination of at least one organic solvent having a boiling point below 100xc2x0 C. with at least one organic solvent having a boiling point of 100xc2x0 C. or higher. Among the above-mentioned organic solvents, those having a boiling point below 100xc2x0 C. include, for example, dimethyl carbonate,and 1,2-dimethoxyetane. Those having a boiling point of 100xc2x0 C. or higher include ethylene carbonate, methylethyl carbonate, diethyl carbonate, and propylene carbonate.
The above-described boiling point means a boiling point at atmospheric pressure. With a solvent having a boiling point below 100xc2x0 C., water is made to vaporize easier as the solvent vaporizes or boils, so that a trace amount of water is removed. With a solvent having a boiling point of 100xc2x0 C. or higher, a trace amount of water can be removed as the solvent vaporizes under heating at about 150xc2x0 C.
By heating the solvent while continuously leading the inert gas through the solvent, a final water content can reach 3 ppm or less. The vaporized solvent may be recovered and used again.
Next, a lithium electrolyte is dissolved while maintaining a temperature of the solvent at 20xc2x0 C. or lower, which is called step (b). Any known lithium electrolyte may be used such as LiPF6, LiClO4, LiBF4, LiAsF6, LiSbF6, LiAlCl4, and LiCF3SO3. Among these, LiPF6 is preferred in battery performance.
The lithium electrolyte is dissolved in a concentration of from about 0.5 to 2.0 moles/liter, preferably 0.7 to 1.5 moles/liter, more preferably 0.8 to 1.2 moles/liter in an atmosphere of an inert gas. In this step, a temperature of the solvent is-maintained at 20xc2x0 C. or lower, preferably 18xc2x0 C. or lower. By doing so, thermal decomposition due to a heat of dissolution can be prevented. The temperature can be maintained at 20xc2x0 C. or lower by controlling the amount of the electrolyte to be added while monitoring the temperature of the solvent, and/or using a known means for cooling such as an electronic cooler.
Preferably, water or acids are further removed by adding such an adsorbent to the obtained electrolytic solution, as molecular sieves, activated carbon, activated alumina, and magnesium oxide. More preferably, following the above step (b), step (c) is performed where at least one lithium compound is added to the electrolytic solution, which compound is selected from the group consisting of lithium amide compounds represented by the formula, LiNR1R2, lithium imide compounds represented by the formula, Li2NR3, lithium borohydride and derivatives thereof represented by the formula, LiBR4R5R6R7, organic lithium compounds represented by R8Li, lithium alkoxides represented by R9OLi, and lithium aluminum hydride and derivatives thereof represented by LiAlR10R11R12R13, wherein each of R1-R13 independently represents hydrogen or a hydrocarbon residue. By adding the lithium compound, not only the acid content is reduced but also additional generation of acids is suppressed, and consequently, the free acid content which has been reduced by the above-described steps (a) and (b) can be maintained for a prolonged period of time.
Preferably, each of R1-R13 is independently selected from the group consisting of hydrogen, alkyl, aryl, and allyl. More preferably, each of R1-R13 is selected from the group consisting of alkyl, aryl, and allyl having 1 to 6 carbon atoms.
Examples of the lithium amid compounds represented by the formula, LiNR1R2, include LiNH2, LiN(CH3)2, LiN(CH3)(C2H5), LiN) (C2H5)2, LiN(n-C3H7)2, LiN(CH(CH3)2)2), LiN(n-C4H9)2, LiN(C5H11)2, LiN(C6H13)2, LiN(C6H11)2, LiN(C6H5)2 and the following compounds: 
Examples of the lithium imide compounds represented by the formula, Li2NR3, include Li2NH, and Li2NCH3.
Examples of the lithium borohydride and derivatives thereof represented by the formula, LiBR4R5R6R7, include LiBH4, LiB(C2H5)3H, and LiB(C4H9)3H.
Examples of the organic lithium compounds represented by R8Li include CH3Li, C2H5Li, n-C4H9Li, s-C4H9Li, t-C4H9Li, (C6H5)3CLi, C6H5CH2Li, (CH3)2NCH2Li, CH2xe2x95x90CHLi, CH2xe2x95x90CHCH2Li, Cl3CLi, C6H5Li and the following compounds: 
An example of the lithium alkoxides represented by R9OLi is C6H5OLi.
Examples of the lithium aluminum hydride represented by LiAlR10R11R12R13 and derivatives thereof include LiAlH4, LiAl(C2H5)3H, and LiAl(C4H9)3H.
These compounds are readily soluble in an organic solvent used for an electrolytic solution and have a high reactivity with water or free acids, while they are inert to the electrolytes or active materials of electrodes.
The present electrolytic solution can be prepared by adding and dissolving at least one of these compounds in an electrolytic solution comprising a lithium~electrolyte in an atmosphere of an inert gas. Its amount to be added is determined as desired, depending on active materials of a battery, composition of the electrolytic solution or the like, but typically is about 1 to 50 molar equivalents, preferably 1 to 25 molar equivalents, and more preferably 1 to 10 molar equivalents, per mole of the total of water and free acids, converted as hydrofluoric acid. If the amount is less than the above-mentioned lower limit, no effect is attained. If the amount is larger than the above-mentioned higher limit, problems may arise, for example, insolubility. Usually, 1.3 to 2.0 molar equivalents, for instance, about 1.5 molar equivalents may be enough to attain the present effects, but a larger amount in the aforesaid range may be added.
The present electrolytic solution is characterized by a water content of 3 ppm or less and a free acids content converted as hydrofluoric acid, of less than 1 ppm. The free acids content may be determined by, for example, neutralization titration in a non-aqueous solvent, as described in detail in the Examples.
A structure of the present lithium battery is not limited and may has any known structure of a lithium secondary battery. As an active material for an anode, for example, lithium metal or carbonaceous materials such as graphite can be used. In the present invention, a carbon electrode is preferred. As an active material for a cathode, metal oxide containing lithium ions such as LiCoC2 and LiNiO2 can be used. The battery may be constructed by placing the active material for a cathode such as LiCoO2, opposite to the carbon electrode with a separator impregnated with the electrolytic solution interposed between the electrodes, and assembling under pressure an anode terminal of a flat can which is made of, for instance, stainless steel expanded metal and a cathode terminal with collectors being interposed between each of the terminals and the electrodes.